Gene transfer is also being utilized to modify the quality of harvested products to maximize their use as food or industrial raw material. In the same manner, attempts to alter the amino acid composition of storage proteins to increase their nutritional value have been reported. These efforts have been met with mixed results.
Transgenic plants are also suitable for producing peptides and proteins used as pharmaceuticals, such as enkephalins, human serum albumins, or interferons. The production in transgenic plants of vaccines for use against various illnesses is being considered to reduce production costs. Additionally, transgenic plants can be engineered to produce a wide array of metabolites. Transgenic plants have been used to produce a variety of metabolites including biodegradable plastics know as polyhydroxyalkanoates, various oils that are useful for both human consumption and industrial purposes and carbohydrates.
Many factors affect gene expression in plants and other eukaryotic organisms. Recently, small RNAs, 21-26 nucleotides, have emerged as important regulators of eukaryotic gene expression. The known small regulatory RNAs fall into two basic classes. One class of small RNAs is the short interfering RNAs (siRNAs). These play essential roles in RNA silencing, a sequence-specific RNA degradation process that is triggered by double-stranded RNA (dsRNA) (see Vance and Vaucheret (2001) Science 292:2277-2280, and Zamore (2001) Nat Struct Biol 8:746-750 for recent reviews on RNA silencing in plants and animals, respectively), RNA silencing plays a natural role in defense against foreign nucleic acids including virus resistance in plants and control of transposons in a number of organisms. siRNAs are double-stranded with small 3′ overhangs and derive from longer dsRNAs that induce silencing. They serve as guides to direct destruction of target RNAs and have been implicated as primers in the amplification of dsRNA via the activity of a cellular RNA dependent RNA polymerase. In plants, si-like RNAs have also been associated with dsRNA-induced transcriptional gene silencing (TGS), a process in which dsRNA with homology to promoter regions triggers DNA methylation and inhibits transcription. The TGS-associated small RNAs, unlike true siRNAs, are not involved in RNA degradation.
The second known class of small RNAs is the small temporal RNAs (stRNAs), lin-4 and let-7, that control certain developmental switches in C. elegans. The stRNAs are single-stranded, although they derive from larger precursor RNAs that are partially double-stranded. StRNAs are also functionally different from siRNAs: they interact with the 3′-untranslated end of target mRNA and inhibit translation rather than mediating RNA degradation. Unlike siRNAs, the stRNAs are only partially complementary to their target mRNAs. Remarkably, hundreds of new stRNA-like small RNAs termed microRNAs (miRNAs have been discovered in worms, flies, humans, and plants and miRNAs are likely to be discovered in other organisms as well. Like stRNAs, the miRNAs are single-stranded, and their accumulation is developmentally regulated. They derive from partially double-stranded precursor RNAs that are transcribed from genes that do not encode protein. Like stRNAs (and unlike siRNAs involved in RNA silencing), most of the miRNAs lack complete complementarity to any putative target mRNA. Although their functions are, as yet, not known, it is hypothesized that they regulate gene expression during development, perhaps at the level of development. However, given the vast numbers of these small regulatory RNAs, it is likely that they are functionally more diverse and regulate gene expression at more than one level.